Field
Aspects of the present invention relate to a solution deposition method for forming a metal oxide or metal hydroxide layer.
Discussion of the Background
Metal oxide and metal hydroxide films are useful for a wide variety of applications and may be deposited by a number of different methods. In many of these cases, the deposition of a metal oxide or metal hydroxide film involves the use of a seed layer or nucleation layer, which is deposited or processed under different conditions or by a different method than the deposition of the bulk of the film. The purpose of a seed/nucleation layer may be to provide a uniform distribution and density of sites for additional material to be deposited during a subsequent growth process, and/or to create a different set of properties at the immediate interface with the substrate than those of the bulk of the film.
When synthesizing a new phase of matter on the surface of an existing phase, the new phase should nucleate before growing. Nucleation of a new phase has a higher energy barrier than continued growth of the new phase. As a result, some conditions that will result in growth on an existing seed layer may not be adequate for nucleation on an unseeded substrate of a different phase, and conditions necessary for nucleation on a substrate may lead to poor quality growth. Therefore, it is often beneficial to split the formation of a film into separate nucleation and growth steps. This has been shown to be especially useful for the growth of metal oxides and metal hydroxides from solution.
Metal oxides and metal hydroxide films can be synthesized by a number of solution-based growth methods including, hydrothermal and solvo-thermal growth, chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), Electroless Deposition, etc. Solution growth methods have been used previously to synthesize a wide variety of films and Micro/Nano-Structures. In many of these cases, the deposition of a uniform film or array of nano/microstructures involves the use of a seed (nucleation) layer.
A seed layer provides a uniform distribution of sites for low-temperature solution growth. Without a seed layer, conditions used for solution deposition/growth typically lead to non-uniform and/or low density distribution of nucleation sites, which develop into a low density of spatially separated structures or “islands”, rather than a desired uniform array or film. Several different methods have previously been explored for seed layer creation, including coating the substrate with a suspension of nanoparticles, coating with a metal-organic precursor film, which, upon heating, decomposes and crystallizes into a metal oxide, vapor deposition of a thin metal oxide layer, and aqueous deposition by initiating the rapid precipitation of a metal oxide from solution.
These techniques all have serious drawbacks, especially for the deposition of epitaxial films. The use of nanoparticle seeds deposited from suspension is not compatible with epitaxy as it typically creates a seed layer with random orientation. The same is true for related art metal-organic precursor film methods, unless very high temperatures are used to recrystallize the seed film. Vapor deposition is capable of producing epitaxial seed layers, but the use of such methods to produce the seed layer negates much of the potential cost or other processing advantages of using low temperature aqueous solution deposition for the subsequent bulk film growth.
Recently, thin films of ZnO and related materials have been demonstrated using a solution process where a precursor solution is prepared by the dissolution of zinc oxide or zinc hydroxide powder in aqueous ammonia. Films were prepared from this type of precursor solution by spin-coating and other coating or printing methods. In the prior art, the resulting ZnO films have been applied to the fabrication thin-film transistors, wherein the substrate for deposition is typically SiO2 or glass. These ZnO films are polycrystalline or amorphous in nature, not epitaxial. A ZnO film deposited by this method forms an entire ZnO layer, and does not serve as a seed or nucleation layer for subsequent solution deposition.